Variables & Strings
Variables are used to store values. A string is a series of characters, surrounded by single or double quotes.
Hello world
print("Hello world!")
Hello world with a variable
msg = "Hello world!"
print(msg)
Concatenation (combining strings)
first_name = 'albert'
last_name = 'einstein'
full_name = first_name + ' ' + last_name print(full_name)
Lists
A list stores a series of items in a particular order. You access items using an index, or within a loop.
Make a list
bikes = ['trek', 'redline', 'giant']
Get the first item in a list
first_bike = bikes[0]
Get the last item in a list
last_bike = bikes[-1]
Looping through a list
for bike in bikes:
print(bike)
Adding items to a list
bikes = []
bikes.append('trek')
bikes.append('redline')
bikes.append('giant')
Making numerical lists
squares = []
for x in range(1, 11):
squares.append(x**2)
List comprehensions
squares = [x**2 for x in range(1, 11)]
Slicing a list
finishers = ['sam', 'bob', 'ada', 'bea'] first_two = finishers[:2]
Copying a list
copy_of_bikes = bikes[:]
Tuples
Tuples are similar to lists, but the items in a tuple can't be modified.
Making a tuple
dimensions = (1920, 1080)
If Statements
If statements are used to test for particular conditions and respond appropriately.
Conditional tests
equals x == 42
not equal x != 42
greater than x > 42
or equal to x >= 42
less than x < 42
or equal to x <= 42
Conditional test with lists
'trek' in bikes
'surly' not in bikes
Assigning Boolean values
game_active = True
can_edit = False
A simple if test
if age >= 18:
print("You can vote!")
If-elif-else statements
if age < 4:
ticket_price = 0
elif age < 18:
ticket_price = 10
else:
ticket_price = 15
Dictionaries
Dictionaries store connections between pieces of information. Each item in a dictionary is a key-value pair.
A simple dictionary
alien = {'color': 'green', 'points': 5}
Accessing a value
print("The alien's color is " + alien['color'])
Adding a new key-value pair
alien['x_position'] = 0
Looping through all key-value pairs
fav_numbers = {'eric': 17, 'ever': 4} for name, number in fav_numbers.items():
print(name + ' loves ' + str(number))
Looping through all keys
fav_numbers = {'eric': 17, 'ever': 4} for name in fav_numbers.keys():
print(name + ' loves a number')
Looping through all the values
fav_numbers = {'eric': 17, 'ever': 4}
for number in fav_numbers.values():
print(str(number) + ' is a favorite')
UserInput
Your programs can prompt the user for input. All input is stored as a string.
Prompting for a value
name = input("What's your name? ")
print("Hello, " + name + "!")
Prompting for numerical input
age = input("How old are you? ")
age = int(age)
pi = input("What's the value of pi? ")
pi = float(pi)
While-Loops
A while loop repeats a block of code as long as a certain condition is true.
A simple while loop
current_value = 1
while current_value <= 5:
print(current_value)
current_value += 1
Letting the user choose when to quit
msg = ''
while msg != 'quit':
msg = input("What's your message? ")
print(msg)
Functions
Functions are named blocks of code, designed to do one specific job. Information passed to a function is called an argument, and information received by a function is called a parameter.
A simple function
def greet_user():
"""Display a simple greeting."""
print("Hello!")
greet_user()
Passing an argument
def greet_user(username):
"""Display a personalized greeting."""
print("Hello, " + username + "!")
greet_user('jesse')
Default values for parameters
def make_pizza(topping='bacon'):
"""Make a single-topping pizza.""" print("Have a " + topping + " pizza!")
make_pizza()
make_pizza('pepperoni')
Returning a value
def add_numbers(x, y):
"""Add two numbers and return the sum."""
return x + y
sum = add_numbers(3, 5)
print(sum)
Class
A class defines the behavior of an object and the kind of information an object can store. The information in a class is stored in attributes, and functions that belong to a class are called methods. A child class inherits the attributes and methods from its parent class.
Creating a dog class
class Dog():
"""Represent a dog."""
def init(self, name):
"""Initialize dog object."""
self.name = name
def sit(self):
"""Simulate sitting.""" print(self.name + " is sitting.")
my_dog = Dog('Peso')
print(my_dog.name + " is a great dog!")
my_dog.sit()
Inheritance
class SARDog(Dog):
"""Represent a search dog."""
def init(self, name):
"""Initialize the sardog."""
super().init(name)
def search(self):
"""Simulate searching.""" print(self.name + " is searching.")
my_dog = SARDog('Willie')
print(my_dog.name + " is a search dog.")
my_dog.sit()
my_dog.search()
I
Reading a file and storing its lines
Working With Files
filename = 'LP.txt'
with open(filename) as file_object: lines = file_object.readlines()
for line in lines:
print(line)
Writing to a file
filename = 'journal.txt'
with open(filename, 'w') as file_object:
file_object.write("I love programming.")
Appending to a file
filename = 'journal.txt'
with open(filename, 'a') as file_object:
file_object.write("\nI love making games.")
Exception Handling
Exceptions help you respond appropriately to errors that are likely to occur. You place code that might cause an error in the try block. Code that should run in response to an error goes in the except block. Code that should run only if the try block was successful goes in the else block.
Catching an exception
prompt = "How many tickets do you need? "
num_tickets = input(prompt)
try:
num_tickets = int(num_tickets)
except ValueError:
print("Please try again.")
else:
print("Your tickets are printing.")
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